Implement the keyboard shortcut editor for the preferences window

[ensonic]

src/ui/edit/settings-page-shortcuts.c has some boilerplate and planning comments.

This needs work on on the settings schema so that we can store the keyboard shortcuts. Alternatively we can store them in the custom css theme.

Ideally each part of the UI that has configurable shortcuts maintains a set of those and registers them 'globally' so that the preferences page just shows all these sections and allows editing them.